This invention relates to a process for the separation of the isotopes of an element, and preferably the isotopes of uranium, utilizing certain compositions of matter containing that element.
The significance of the present invention primarily arises from the fact that for years, those skilled in this art have tried to take advantage of the known fact that the absorption spectra of atoms or molecules of a given element exhibit an isotopic shift, and that it should therefore be possible to excite isotopes or moieties containing isotopes of those elements with light of a selected wavelength. Until quite recently, however, the actual application of this principle has proven quite difficult, primarily because the particular absorption lines involved were located at wavelengths requiring the use of light sources which are not commercially available or economically feasible, because the particular atoms or molecules in question were not readily attainable in the vapor phase at reasonable operating temperatures, or because the particular isotopic shift in question exhibited overlapping bands, thus rendering selective excitation considerably more difficult. In U.S. Patent application Ser. No. 865,963, filed on Dec. 30, 1977 in the names of Messrs. Martin B. Dines, Richard B. Hall, Andrew Kaldor, George M. Kramer and Edward T. Maas, Jr., however, these problems have been solved by the utilization of volatile uranyl compounds such as those having the general formula UO.sub.2 (1,1,1,5,5,5-hexafluoroacetylacetonate)*.sub.2.sup.. L where L is a neutral ligand. By employing such compounds a highly useful isotope separation process has therefore been demonstrated in that application. FNT *(hfacac)
In the background section of U.S. Patent application Ser. No. 865,963, a number of references teaching various compounds having the general formula UO.sub.2 (.beta.-diketonate).sub.2.sup.. L, where L is a neutral ligand, are disclosed. These include U.S. Patent Appication Ser. No. 662,600 of Messrs. Schlessinger and Brown published in the Official Gazette on Mar. 6, 1951, Chemical Abstracts, 46, 10192b, as well as those same author's subsequent publication in the Journal of the American Chemical Society, 75, pages 2446-8 (1953) in which they disclose UO.sub.2 (1,1,1-trifluoroacetylacetone).sub.2 having the highest vapor pressure for any of the .beta.-diketones which they studied, namely about 0.0027 torr at 130.degree. C.
In the aforesaid application of Messrs. Dines et al., reference is also made to a comprehensive review of the properties of various uranyl compounds with chelating ligands namely Casellato et al, in Inorganica Chemica Acta, 18, 77-112 (1976). In that article the behavior of the actinides in their various oxidation states and combined with various organic chelating ligands such as the .beta.-diketones is discussed in detail.
Reference is also made in the Dines et al. application to Subramanian et al, "Complexes of Uranyl .beta.-Diketones with Aromatic Amine N-Oxides", Journal of Inorganic Nuclear Chemistry, 33, 3001 (1971) which discussed a number of compounds of the general formula UO.sub.2 (1,1,1,5,5,5-hexafluoroacetylacetonate).sub.2.sup.. L where the L ligands are various amine N-oxides, such as pyridine N-oxide. It is also noted that the same types of compounds, but where L is a sulfoxide or a phosphine oxide, are disclosed in articles such as Sieck, "Gas Chromatography of Mixed-Ligand Complexes of the Lanthanides and Related Elements" submitted for his Ph.D. thesis, Iowa State University, 1971 and two other articles by Sieck in Chemical Abstracts, 75, 147395Q and Nuclear Science Abstracts, 25, (17), 39410 (1971). Also, Mitchell (Synergic Solvent Extraction and Thermal Studies of Fluorinated Beta-Diketone-Organophosphorous Adduct Complexes of Lanthanide and Related Elements, Ph.D. Thesis, Iowa State University, 1970) prepared the tributylphosphate complex of UO.sub.2 (hfacac).sub.2, and showed that it sublimed at about 150.degree. C.
In another co-pending application, Ser. No. 961,363, filed on Nov. 16, 1979, by Messrs. Hall, Kaldor, Kramer and Dines, a number of volatile uranyl compounds are also disclosed including those having the general formula UO.sub.2 AA'L.sub.n, where A and A' are certain selected anions and L is again a neutral ligand. Both of the above-noted co-pending applications of Messrs. Dines et al. and Hall et al. discuss an article by Belford et al. (J. Inorg. Nucl. Chem. 14, 169 (1960) in which the authors describe their prepraration of UO.sub.2 (hfacac).sub.2 tetrahydrate, which they describe as decomposing upon heating above 58.degree. C. This article then goes on to discuss the infrared absorption bands for various uranyl compounds, and the effect of ligand substitution on the visible spectra, concluding that the more basic ligands attach more securely to the uranium atom, thus decreasing its coordinating tendencies.
A number of references have also discussed compounds variously described as uranyl phthalocyanine (Bloor et al., Canadian Journal of Chemistry, 42, 2201-2208 (1964)), said to be sublimable under a vacuum "below 0.01 mm pressure at 400.degree.-450.degree. C." and uranyl superphthalcyanine (Day, Marks and Wachter, "Large Metal Ion-Centered Template Reactions. A Uranyl Complex of Cyclopentakis(2-iminoisoindoline)": J.A.C.S., 97:16, Aug. 6, 1975, 4519-4527). Furthermore, U.S. Pat. No. 4,097,384 to Messrs. Coleman and Marks discloses the possible laser irradiation of that compound, as well as other possible uranyl compounds.
In a recent publication (within the last year, but not prior to the applicants' invention) the authors discuss the quadridentate complex UO.sub.2 (.beta.-diketone).sub.2 ; Ekstrom et al., "The Preparation and Properties of Bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato) dioxouranium (VI)", Inorg. Nucl. Chem. Letters, Volume 14, pages 301-304, Received Apr. 13, 1978, received for publication July 5, 1978. In their experimental procedures the authors claim to produce a compound having the formula UO.sub.2 (hfacac).sub.2 which is deep red, crystalline material, melting without decomposition at 183.degree. C. in an argon atmosphere. They further suggest that from their measurements the UO.sub.2 (hfacac).sub.2 is largely dimeric in the gas phase, and is trimeric in the solid state. They also disclose that the IR spectrum of this compound now has three partly overlapping bands with maxima at 950 cm.sup.-1, 936 cm.sup.-1, and 920 cm.sup.-1 instead of a single sharp band near 950 cm.sup.-1. Other articles by some of the same authors as the Ekstrom et al article appeared in Inorganic Chemistry, Volume 17, No. 11, received Apr. 17, 1978, pages 3285-3289, which discusses the crystal structure of the solid phase, allegedly that of the trimer [UO.sub.2 (hfacac).sub.2 ].sub.3, and in the Journal of Physical Chemistry, Volume 82, No. 20, received Apr. 24, 1978. In the latter article the dimeric gaseous phase in compounds such as UO.sub.2 (hfacac).sub.2 is discussed.
Finally, in an article entitled "Photoelectron Spectroscopy of f-Element Coordination Compounds" by Messrs. Fragala et al. in Inorganic Chemistry, Volume 17, No. 11 received Mar. 10, 1978 the authors also disclose the results of their study of compounds such as UO.sub.2 (hfacac).sub.2. In particular, these authors studied the photoelectron spectra of such compounds in order to gain insight into the nature of the bonding in the uranyl ion. The dimerized UO.sub.2 (hfacac).sub.2 molecule is also discussed in the aforesaid Dines et al. and Hall et al. applications.
In Dines et al., Ser. No. 865,963, it is noted that "in the absence of an appropriate stabilizing neutral ligand it is impossible to generate the stable monomeric vapor of the uranyl (hfacac).sub.2 which would be necessary for an isotope separation process." And, in Hall et al it is further stated that with no other molecules being present (such as the ligand L) " . . . UO.sub.2 (hfacac).sub.2 will dimerize, sharing two of the oxygens, and thus each UO.sub.2.sup.+2 ion will have the requisite five oxygens around it." This application goes on to note, however, that "this dimer is not desired for laser isotope separations (monomers are preferred) because its volatility is too low and it allows for scrambling of absorbed energy and reduction in selectivity. Energy absorbed by the selectively excited UO.sub.2.sup.+2 group is diluted due to enhanced transfer to the second UO.sub.2.sup.+2 group which is intimately bound to it in a dimer or oligomer." It is therefore concluded that judicious selection of a neutral Lewis based molecule must be made to stabilize the UO.sub.2 (hfacac).sub.2 as a monomer.